Voltammetric and spectroscopic study of ferrocene and hexacyanoferrate and the suitability of their redox couples as internal standards in ionic liquids

Abstract

Ionic liquids and deep eutectic solvents have great potential in metallurgical applications as specialised solvents. In order to design ionometallurgical electrowinning and electrorefining processes, it is essential to characterise the electrochemical behaviour of metal complexes and compare potentials between relevant solvents. For such investigations, a universal reference redox couple would be desirable. In this study we investigate the speciation and electrochemical behaviour of ferrocenium/ferrocene and hexacyanoferrate(iii/ii) as possible reference couples for 15 different ionic media on platinum (Pt), glassy carbon (GC) and gold (Au) working electrodes. Amongst other parameters, formal electrode potentials, charge transfer coefficients, and rate constants were calculated. It was found that neither ferrocene nor hexacyanoferrate are universally suitable as redox standards in the liquids investigated. Nevertheless, hexacyanoferrate exhibits clear advantages in most of the strongly coordinating ionic liquids studied here.

Electron Transfer Kinetics in a Deep Eutectic Solvent

“…In the framework of the Marcus-Hush theory, 4,5 the electron transfer kinetics is for a large part driven by the dynamics and solvent reorganization that accompany the electrochemical process. 6,7,8,9 This explains that numerous publications have addressed this question in ionic liquids (see for example references 10,11,12,13,14 ) and more recently in DESs. 14,15,16,17,18,19,20 In ionic liquids, the electron transfer of a redox couple is considerably decreased when passing from a classical organic solvents to a ionic liquid, generally by a ratio 10-100 depending on the outer-sphere character of the electron transfer.…”

“…6,7,8,9 This explains that numerous publications have addressed this question in ionic liquids (see for example references 10,11,12,13,14 ) and more recently in DESs. 14,15,16,17,18,19,20 In ionic liquids, the electron transfer of a redox couple is considerably decreased when passing from a classical organic solvents to a ionic liquid, generally by a ratio 10-100 depending on the outer-sphere character of the electron transfer. [8][9][10][11][12][13] Concerning the DES, well-defined cyclic voltammograms in Ethaline or Reline were reported for "outer-sphere redox" couple like ferrocene/ferrocenium [14][15][16][17]19 but also for highly charged couple like ferro/ferricyanide, 14,19 suggesting fast electron transfers.…”

“…14,15,16,17,18,19,20 In ionic liquids, the electron transfer of a redox couple is considerably decreased when passing from a classical organic solvents to a ionic liquid, generally by a ratio 10-100 depending on the outer-sphere character of the electron transfer. [8][9][10][11][12][13] Concerning the DES, well-defined cyclic voltammograms in Ethaline or Reline were reported for "outer-sphere redox" couple like ferrocene/ferrocenium [14][15][16][17]19 but also for highly charged couple like ferro/ferricyanide, 14,19 suggesting fast electron transfers. However, recent reports about electron constants k s were reported to be significantly lower in DES (below 10 -3 cm s -1 ) than in molecular solvents and in the same range or below the values reported in ionic liquids.…”

Electrochemical Characterization of TEMPO Radical in Ionic Liquids

“…S4) probably because the TEMPO absorbed onto the surface of glassy carbon electrode was reduced to forming insoluble solid salt with EMI-TFSI. 23 The effect of working electrode materials was also studied. When platinum (Pt) was used as the working electrode, the peak currents for n-type TEMPO reaction decreased with increases in the peak separation width as shown in Supporting Fig.…”

Replicating Bulk Electrochemistry in Liquid Cell Microscopy

“…In the former case ( Figure 1B), the copper deposition and stripping occurs at the Pt working electrode at distinct redox peaks and the result mimics the bulk electrochemical cells with large electrode areas and larger volume of electrolyte solution [6]. In the latter case ( Figures 1C and 1D), the redox reaction of 20 mM ferrocyanide/ 20 mM ferricyanide in 0.1M KCl at different voltage scans show reversible electrode reaction during both the forward and reverse scans, elucidating bulk behavior [7]. This work highlights the potential sources of electrochemical measurement artefacts in liquid TEM and proposes recommendations to minimize them.…”